A system is being developed which will control phasic aortic pressure in awake closed chest experimental dogs. Aortic pressure can either be manipulated on a beat to beat basis in an arbitrary manner, or a waveform can be captured and then reproduced over an extended period of time. A typical use of the system will involve evaluating the effects of a drug intervention where the pressure waveform before the intervention is reproduced after the intervension, thus allowing changes in inherent muscle contractility to be observed apart from changes in systemic pressure. A modification of the system will be developed to control end diastolic filling pressure. Aortic pressure is manipulated by a feedback controlled gas filled intra-aortic balloon catheter which resides in the descending aorta and is used in conjunction (when necessary) with a speed controlled roller blood pump. The pump bypasses the systemic circulation via catheters to provide the DC and low frequency control whereas the rapidly responding baloon provides the high frequency control. Aortic pressure at the outlet of the heart is measured with a catheter tip pressure transducer and fed back via an electronic controller to move a linear motor which positions a bellophragm which in turn inflates or deflates the balloon. The hardware has been bench tested and will shortly be evaluated in experimental dogs.